You can make use of Interface over here.
Create an interface say "I" and implement that interface in all the 3 classes A,B,C.
After that in the method Test you can accept "I" and in the list also you can use "I", like..
public void test(I interfaceImplementedObject)
{
..........
..........
List<I> rangePlants = something;
..........
..........
}